Instructor Dashboard v2 (disabled)

This is a refactor and reorganization of the existing instructor dashboard. The previous dashboard still works and its view has been moved to legacy.py
The v2 dashboard is disabled by default under the ENABLE_INSTRUCTOR_BETA_DASHBOARD feature flag. When enabled, it is linked to from the current dashboard.

Features:
The v2 dashboard has reached almost-parity.
To get a sense for what is implemented, see these screenshots
https://edx-wiki.atlassian.net/wiki/pages/viewpage.action?pageId=27329769

Testing:
The views and json api are NOT tested. The views will definitely change soon (v3), the json api may change.
Modules one level back from views and api have unit tests.
@wedaly does this seem like the right approach?

This is a wiki page with a few notes on enabling the dashboard and its module organization.
https://edx-wiki.atlassian.net/wiki/pages/viewpage.action?pageId=28049502

Feature flag:
The purpose of this pull request is to socialize the v2 dashboard to other people working on the dashboard ( @brianhw )
Below are tasks to be done before flipping the switch on the v2 feature flag
- minor ux fixes
- instrument (segment.io) with @rlucioni 
- final product review

@jkarni , I addressed many of your comments on the other PR (https://github.com/edx/edx-platform/pull/279).

Reviewers:
@wedaly 
@markchang